1. Field of the Invention
The disclosure relates generally to medium access control with adaptive power management methods and systems, and, more particularly to methods and systems for use in wireless networks such as ad-hoc wireless networks, where stations adaptively determine the awake/sleep schedule according to residual power state, quality-of-service requirements, or other considerations, and transmit data accurately.
2. Description of the Related Art
Currently, IEEE 802.11 is the most popular international medium access control (MAC) standard for WLANs (Wireless Local Area Networks). Based on the network architecture, wireless networks can be approximately divided into: infrastructure WLANs and ad hoc networks. FIG. 1 is a schematic diagram illustrating an ad hoc network. As shown in FIG. 1, each station (110, 120, 130, 140 and 150) can dynamically communicate with adjacent stations for data transmissions.
FIG. 2 is a schematic diagram illustrating a power consumption model for a general wireless network interface card (or adapter). Each station can stay in one of transmission, reception, listen, or doze states. As shown in FIG. 2, power consumption is approximately between 1.6 W and 1.2 W when the station is in either the transmission, reception, or listen states, but close to zero when in the doze state. In IEEE 802.11 power management for ad hoc networks, time is divided into fixed-sized BIs (Beacon Intervals), each of which contains an ATIM (Announcement Traffic Indication Message) window. Each station in a power saving (PS) mode (or “power-saving station”) must wake up at the beginning of each BI and remain awake in the ATIM window, awaiting the ATIM frame from other stations. If no ATIM frame is received in the ATIM window, then that station may enter a doze state after the ATIM window ends. If an ATIM frame is received in the ATIM window, then the station returns the ATIM ACK (Acknowledgement) to the station transmitting the ATIM frame, and remains awake after the ATIM window ends. After the end of the ATIM window, the station sending ATIM frames uses the DCF (distributed coordination function) procedure to transmit the buffered data frames to its intended destination, and the destination acknowledges receipt. For a more detailed presentation, please refer to IEEE 802.11 specification.
FIG. 3 is a schematic diagram illustrating an example of power management in an ad hoc network based on IEEE 802.11. As shown in FIG. 3, when a BI 1 begins (the timing is referred to as TBTT (Target Beacon Transmission Time)), stations X and Y compete to transmit a beacon frame for timing synchronization. It is understood that, in the example of FIG. 3, station X transmits a beacon frame for timing synchronization between stations comprising station X in the network. Since no ATIM frame is received in the ATIM window (AW for short), both stations X and Y enter the doze state (S) after the AW ends. BI 2 begins, and station X successfully transmits a beacon frame. Since station X receives an ATIM frame A from station Y in the AW of BI 2, station X returns a ATIM ACK a to station Y, and remains awake after the AW ends. After the AW ends, station Y can transmit a data frame D to station X, and station X returns a data ACK d to station Y after receiving the data frame D.
As described, in IEEE 802.11, each station in power saving mode must wake up in the ATIM window of “every” BI even if battery power is low or there is no traffic for it. Hence there is a need for each power saving station to dynamically tune its listen interval (the number of BIs between two adjoining AWs) according to the remaining battery power status or other QoS considerations. Obviously, the LI value is fixed at “one” in IEEE 802.11. In the invention, the LI of a power saving station can be adjusted according to parameters of quality of service or the remaining power of the station, substantially reducing power consumption on station.